The present invention relates to gamma ray detectors. More specifically, the present invention is related to light weight directional gamma ray detectors having a high degree of angular resolution, high efficiency, and an ability to discriminate low energy gamma rays.
Conventional directional gamma ray detectors generally employ the use of extended collimators and shielding to channel gamma rays from a predetermined direction to the measuring device of the gamma ray detector. An example of a conventional direction gamma ray detection system is illustrated in U.S. Pat. No. 3,562,526 issued to Lawson. The detection system disclosed in this reference employs a shield having a plurality of passages extending in parallel relationship with one another to collimate incoming gamma rays, and requires a great deal of space and weight due to the extensive use of shielding material and the construction of the collimator. The detection system disclosed in U.S. Pat. No. 3,562,526, therefore, is impractical in the many applications where space or weight is a concern.
Another type of conventional directional gamma ray detectors is illustrated U.S. Pat. No. 4,136,282 issued to Cox et al. The detector disclosed in U.S. Pat. No. 4,136,282 employs a radiator strip contained within a case that is made of an electrical conductor of a high atomic number that is backed by a conducting strip of a low atomic number. In operation, gamma rays incident normal to the broad side of the radiator will pass through the junction of the radiator strip and conducting strip causing electrons to be freed as a result of three processes, the photoelectric effect, the Compton effect and pair production. The electrons produced by the photoelectric effect in the radiator strip and conducting strip are emitted from the radiator strip and collected by the case. The current in the case is measured to determine the magnitude of incident gamma rays.
The above-described detector, while having a degree of directional sensitivity, does not provide a high degree of angular resolution, i.e., an angular resolution on the order of approximately 0.1 radian or less. In addition, the above-described detector does not discriminate low energy gamma rays, that is, gamma rays below 1 MeV. Since environments where a gamma ray detector may be used frequently have a large number of low energy background gamma rays, discrimination of these low energy gamma rays is essential in order to eliminate unwanted detection signals. Other conventional directional gamma detectors, for example as disclosed in U.S. Pat. No. 4,259,575, also suffer from the lack of a high degree of angular resolution. Thus, extensive collimators or shielding are still needed in order to obtain a high degree of angular resolution, thereby adding to the weight and physical size of the detector.
Thus there is a need for a lightweight, compact and highly sensitive directional gamma ray detector which has a high degree of angular resolution and which can function effectively in an environment having a large number of low energy background gamma rays.